1. Field of the Invention
This invention relates to a feed structure for a dielectrically-loaded antenna and to a method of producing a dielectrically-loaded antenna.
2. Discussion of the Related Art
British Patent Applications Nos. 2292638A and 2310543A disclose dielectrically-loaded antennas for operation at frequencies in excess of 200 MHz. Each antenna has two pairs of dielectrically opposed helical antenna elements which are plated on a substantially cylindrical electrically insulative core made of a material having a relative dielectric constant greater than 5. The material of the core occupies the major part of the volume defined by the core outer surface. Extending through the core from one end face to an opposite end face is an axial bore containing a coaxial feeder structure comprising an inner conductor surrounded by a shielded conductor. At one end of the core the feed structure conductors are connected to respective antenna elements which have associated connection portions adjacent the end of the bore. At the other end of the bore, the shield conductor is connected to a conductor which links the antenna elements and, in these examples, is in the form of a conductive sleeve encircling part of the core to form a balun. Each of the antenna elements terminates on a rim of the sleeve and each follows a respective helical path from its connection to the feed structure.
British Patent Application No. 2367429A discloses such an antenna in which the shield conductor is spaced from the wall of the bore, preferably by a tube of plastics material having a relative dielectric constant which is less than half of the relative dielectric constant of the solid material of the core.
Dielectrically-loaded loop antennas having a similar feed structure and balun arrangement are disclosed in GB2309592A, GB2338605A, GB2351850A and GB2346014A. All of these antennas have the common characteristic of antenna elements on the outside of the core which are top-fed from a coaxial feed structure passing through an axial bore in the core. The balun provides common-mode isolation of the antenna elements from apparatus connected to the feeder structure, making the antenna especially suitable for small handheld devices.
Hitherto, the feed structure has been formed in the antenna as follows. Firstly, a flanged connection bush, plated on its outer surface, is fitted to the core by being placed in the end of the bore where the feed connection is to be made. Then, an elongate tubular spacer is inserted into the bore from the other, bottom, end. Next, a coaxial line of predetermined characteristic impedance is trimmed to length and an exposed part of the inner conductor at one end is bent over into a U-shape. The formed section of coaxial cable is inserted into the bore and the elongate tubular spacer from above and the entire top connection is soldered in two soldering steps: (a) soldering of the inner conductor bent portion to connection portions of the antenna elements on the top face of the core, and (b) soldering of the flanged bush to the shield conductor and to further antenna element connection portions on the top face of the core. The core is then inverted and a second plated bush is fitted over the outer shield conductor of the cable where it is exposed at the opposite end of the core from the bent section of the inner conductor so as to abut the plated bottom end face of the core. Finally, this second bush is soldered to the outer shield conductor and to the plated bottom end face of the core.